The present invention relates to a dynamic pressure type fluid bearing device for use in the rotary head cylinder of a disk driving apparatus and a video tape recorder and capable of rotating the rotary head cylinder at a high speed.
The miniaturization and functionality of electrical machinery and equipment have increased remarkably in recent years. To promote this trend, it is necessary that a video tape recorder and a disk driving device be provided with a bearing capable of rotating at a high speed in the main shaft of the rotary head cylinder thereof. To this end, a dynamic pressure type fluid bearing device is used as the bearing.
Examples of conventional dynamic pressure type fluid bearing devices are described with reference to the drawings.
Referring to FIG. 4 which is a sectional view showing a conventional dynamic pressure type fluid bearing device, a shaft 41 is rotatably fitted into a sleeve 42. A thrust plate 43, fixed to an end of the sleeve 42, is in contact with an end surface 41A of the shaft 41. The thrust plate 43 is provided with a dynamic pressure generating groove 43A containing a lubricant 44. Thus, a thrust bearing is constituted. Dynamic pressure generating grooves 42A and 42B are formed on the inner peripheral surface of the sleeve 42 or the peripheral surface of the shaft 41. The grooves 42A and 42B contain a lubricant 44, respectively. Thus, a radial bearing is constituted. As shown in FIG. 1, the grooves 42A and 42B make acute angles of .theta..sub.1 and .theta..sub.2 generally ranging from 25.degree. to 40.degree. with the rotational direction of the shaft 41 or the sleeve 42. Vents 42C and 42D are formed through the sleeve 42.
The operation of the dynamic pressure type fluid bearing device of the above construction is described below. When a motor not shown is energized, the shaft 41 or the sleeve 42 to which the thrust plate 43 is fixed starts rotating. The lubricant 44 generates pressure due to the pumping operation of the grooves 41A, 42A and 42B. There is no contact between the shaft 41 and the sleeve 42 as well as the thrust plate 43 while they are rotating. As disclosed in Japanese Laid-Open Patent Publication No. 60-78106, the vent 42C is provided to discharge air from a void 42E when the dynamic pressure type fluid bearing device is assembled by fitting the shaft 41 into the sleeve 42. The vent 42D is provided to communicate outside air and air in a void 42F with each other when the volume of the air in the void 42F changes. Thus, the pressure of the void 42F does not change.
In view of a growing demand in recent years for the development of a dynamic pressure type fluid bearing device having a sleeve and a shaft capable of rotating at a higher speed, the above structure has the following disadvantages. That is, the lubricant 44 introduced into the grooves 42A and 42B is discharged from the vent 42D of the sleeve 42 by the centrifugal force when the rotating member is rotating at a high speed, thus resulting in a shortage of the lubricant 44. Consequently, the rigidity of the sleeve 42 or the shaft 41 become insufficient and the shaft 42 rotates off its axis. That is, the rotating members are seized to each other so as not to operate normally.
In recent years, a disk driving device has a very dense structure and has become compact. Therefore, a highly accurate disk driving device having the following structure is in demand: in the main shaft for rotating the disk, a radial bearing having a short span and in addition is thin, and the shaft is off its axis not more than one micron.
Another example of a conventional fluid bearing device is described with reference to FIGS. 5 and 6 which are sectional views thereof. A sleeve 12 is fixed to a chassis 11 with screws 21. A shaft 13 is rotatably fitted into the sleeve 12. The bottom end 13B of the shaft 13 contacts a thrust bearing 14 and is subjected to a thrust load, thus constituting a pivot bearing. The thrust bearing 14 is supported with a thrust receiving plate 15 fixed to the sleeve 12 with the screws 21. The shaft 13 has a hub 16 and a rotor 17 fixed thereto in the vicinity of the top end 13A of the shaft 13. Therefore, the hub 16 and the rotor 17 rotate together with the rotation of the shaft 13. A vent 12B is formed through the sleeve 12 and a herringbone dynamic pressure generating groove 12A is formed on the peripheral surface of the shaft 13 or the inner peripheral surface of the sleeve 12. The groove 12A generally makes an an angle .beta. of approximately 30.degree. with the rotational direction of the shaft 13 and the depth thereof is almost the same as the 2-3 microns interval between the shaft 13 and the sleeve 12 in the radius direction of the shaft 13. Lubricants 18A and 18B mainly composed of mineral oil are provided in the space between shaft 13 and the sleeve 12 and the space between the shaft 13 and the thrust bearing 14, respectively. A rotary disk 19 is mounted over the hub 16.
The operation of the fluid bearing device of the above structure is described below. When a motor not shown is energized, the rotor 17, the hub 16, and the shaft 13 start rotating. As a result, the lubricant 18a generates pressure due to the pumping operation of the groove 12A, and the shaft 13 rotates without contacting the sleeve 12 and the thrust bearing 14.
The above structure, however, has the following disadvantages.
The first is that as shown in FIG. 6, without the sleeve 12 being provided with the vent 12B, the temperatures of the sleeve 12 and the periphery thereof rise when the shaft 13 is rotating at a high speed. As a result, air 20 which has penetrated into the bearing device thermally expands and the air 20 presses the lubricant 18A which has adhered to the shaft 13 outside as shown by 18C in FIG. 6. Thus, the amount of the lubricant 18A becomes less than desired. Additionally, since the diameter of the vent 12B is small, it takes much time to form the vent 12B in the sleeve 12 with a drill, resulting in low productivity.
The second is concerned with the configuration of the shaft 13. Referring to FIG. 5, in order to prevent the abrasion between the bottom end 13B of the shaft 13 and the thrust bearing 14, it is necessary that the bottom end 13B be made of hard stainless steel and the radius of the spherical bottom end surface 13B be great, for example, 5 to 6 mm. The thrust bearing 14 is made of silicon carbide or silicon nitride. The lubricants 18A and 18B consist of viscous mineral oil of approximately 100 centistokes at 20.degree. C. The friction torques of the radial bearing and the thrust bearing 14 become very great at a temperature as low as 0.degree. . The radius of the spherical top end surface 13A of the shaft 13 is required to be small, for example, approximately 3 mm so as to reliably connect the rotary disk 19 with the bearing device. That is, the radii of the two spherical surfaces are different from each other. Therefore, this fluid bearing device has a disadvantage in that it takes time and labor to correctly distinguish both members 13A and 13B from each other in the process of mass production.